A prescription, often abbreviated ℞ or Rx, is a formal communication from physicians or other registered healthcare professionals to a pharmacist, authorizing them to dispense a specific prescription drug for a specific patient. Historically, it was a physician's instruction to an apothecary listing the materials to be compounded into a treatment—the symbol ℞ (a capital letter R, crossed to indicate abbreviation) comes from the first word of a medieval prescription, Latin recipe (lit. 'take thou'), that gave the list of the materials to be compounded. For a communication to be accepted as a legal medical prescription, it needs to be filed by a qualified dentist, advanced practice nurse, physician, or veterinarian, for whom the medication prescribed is within their scope of practice to prescribe.

Source: Wikipedia